In multiple channel systems, channel dynamics can be coupled to one another and be independent of the remaining flow loop or one unstable channel may dominate the transients and make the multiple parallel channel system behave as one single channel. In the single channel case, the whole loop participates in the flow oscillations, whereas in a two parallel channel system the flow oscillations occur in the channels and can be out-of-phase. Previous studies indicated the instabilities are due to condensation in a low pressure natural circulation two parallel channel loop at low powers and inlet subcoolings. A thermal hydraulics computer code was developed to simulate the geysering instability and is based on integral methods where local properties are based on local pressures and the vapor generation model is adjusted accordingly to reflect the vapor generation rate necessary to initiate the instability. This is an important modeling feature since local vapor generation rate depends on local saturation temperature. The code is designed to simulate startup transients in two heated parallel channel natural circulation systems with subcooled water as an initial condition. The numerical simulations illustrated flow reversal in the individual channels, which is a thermal hydraulic feature of condensation-induced geysering.